1. Field of the Invention
The present invention relates generally to welded aluminium alloy members and more particularly to a welded structural member comprising two or more parts of different alloys and/or tempers. These structural members may be used, for example, in aircraft construction.
2. Description of Related Art
It is generally known that when manufacturing semi-finished products and structural elements for aeronautical construction, certain required properties generally cannot be optimized at the same time independently of one another. This is sometimes the case with respect to “static mechanical properties” (particularly the ultimate tensile strength UTS and the yield stress YS), on the one hand, and properties relating to “damage tolerance” (particularly toughness and resistance to fatigue crack propagation) on the other hand. For each contemplated use of a structural element, a proper balance between static mechanical properties and damage tolerance properties needs to be defined. This is known as “property balance”. Additional properties, such as corrosion resistance, can also be included in the property balance, as required, and in some cases, it may even be necessary to define a balance of two or more properties within the static mechanical properties or within the damage tolerance properties, such as yield strength and elongation which tend to be conflicting. The need for optimizing the property balance is particularly strong with respect to parts or structural members where the best results are obtained when static mechanical properties are optimized on one geometric end and damage tolerance is optimized on the other geometric end. Typically, top skin covers are optimized for static mechanical properties and bottom skin covers are optimized for damage tolerance. During the past few years, new alloys have been developed for top and bottom skin panels and the difference of properties between the panels is getting more and more pronounced. In spars, which are connected to both the top skin panel and the bottom skin panel, it would be of great interest to optimize the upper part for static mechanical properties and the bottom part for damage tolerance. However, since spars are made from one alloy in one temper for unwelded integral structures, a choice has to be made as to which properties to optimize. Currently 7040 in T76 or T74 temper is typically chosen to take advantage of the best compromise between the yield stress and toughness offered by this alloy. In order to separately optimize the upper and lower part of, for example, a spar, it has been proposed to manufacture the spar from two different materials. A mechanically fastened structure can be made, but is expensive. It has been proposed to join the structure by welding.
Among welding techniques, two main families may be distinguished. In fusion welding processes, such as resistance spot welding, flash butt welding, laser welding, and arc welding electron-beam welding, the weld is made in the liquid phase above the melting point. In friction welding, where relative movement of the parts to be welded generates heat for joining and in friction stir welding, where a non-consumable rotating tool moves along the joint between two components to generate heat for joining, the weld is made below the melting point, in the solid phase.
Whatever welding technique is used, heat is generated and welding has usually a detrimental effect on the properties of the different materials by decreasing strength in the weld as well as in the zone adjacent to the weld referred to herein as the “heat-affected zone”. However, fusion welding techniques do behave differently and many alloys, such as most 2XXX and 7XXX aluminium alloys series, that cannot be welded reliably by fusion welding techniques, may be joined by friction welding or friction stir welding.
WO 98/58759 (British Aerospace) discloses a method for forming airframe components by joining two components by friction stir welding. Structural airframe components such as wing skin panels, wing spars, and wing skin-spar-ribs may be obtained with the method in this patent application. Hybrid billets of aluminium alloys comprising for example a 7000 series alloy friction stir butt welded to 2000 series, are described. However, no indication is provided in this patent application on how to solve the metallurgical difficulties associated with friction stir welding of age-hardenable aluminium alloys. In particular, the low strength and low corrosion resistance of the heat-affected zone are not discussed.
U.S. Pat. No. 6,168,067 (McDonnell Douglas Corporation) teaches a method to reduce material property degradation during friction stir welding. In particular, the friction stir welding operation is carried out after solution heat treating and quenching and before aging. Aluminium-zinc alloys are not mentioned as examples of binary or ternary alloys useful for the invention. The patent does not mention the possibility of joining different alloys or members from the same alloy in different tempers. The thermomechanical treatment carried out before joining is the same for both pieces to be joined. Aging before joining the pieces by friction stir welding is not mentioned, and the structural members are in a non-equilibrium state during the friction stir welding operation.
EP 0 995 511 (Alcoa) describes how to join layered materials before a combined deformation. The means for attaching the materials is directed to maintain the positioning upon combined deformation, which is a different objective from the present invention.
U.S. 2004/0056075 (Universal Alloys) describes a method to improve strength properties in the heat-affected zone and in the weld zone. The precipitation hardenable aluminum alloy members to be welded are subjected to the following steps: a first aging step, a welding step and a second aging step. The members to be joined are made of the same aluminium alloy and undergo the same first aging treatment before welding. This patent application does not address the particular problems related with joining members of different alloys or members from the same alloy in different tempers
In U.S. Pat. No. 6,802,444 (NASA), a solution to improve the heat treatment of friction stir welding materials is proposed. An aluminium-zinc alloy is first heat treated, then air cooled to room temperature, then assembled by friction stir welding, then solution heat treated for a second time, then quenched and finally aged.
In JP 2000-237882 (Sky Aluminium), friction stir welding is used to join super-plastic aluminium alloys such as an Al—Mg alloy, an Al—Zn—Mg alloy, an Al—Zn—Mg—Cu alloy, an Al—Cu alloy, an Al—Li alloy, an Al—Mg—Si alloy, and an Al—Si alloy. These alloys have a limited grain size, preferably <=30 μm. The heat treatments carried out are specific to super-plastic aluminium alloys.
There is clearly a need for a method capable of providing a solution to the problem, (referred to hereafter as “the problem”), of welding two or more aluminium alloy parts displaying different property balances without significantly damaging properties of the aluminium alloy parts such as static mechanical properties and/or damage tolerance and/or corrosion resistance in the weld, in the heat-affected zone and/or in zones that are not affected by welding.